In the presence of suitable inhibitors, the experimental results show that free radical OH was generated in the H2O-Mn(II)-ethylenediamine-indigocarmine-H3BO3-H2O2 system. Hydroxyl radical OH has hight oxidation capability and its reaction rate constant with a substrate as indigocarmine kOH+Ind was determined by using the concurent reactions. The calculations were done by method of least square after programming on PC and by graphic method.